1. Field of the Invention
The present invention relates to photolithography of semiconductor devices, and more particularly, to a method of making and packaging reticles resulting in reduced particle contamination.
2. Background Information
Photolithography is a process that is commonly used in the manufacture of integrated circuits. The process involves the deposition of a photoresist layer onto an underlying substrate layer. The photoresist is then selectively exposed to light, which chemically alters the photoresist. The photoresist is then developed and those portions of the photoresist that are exposed to light are either hardened or softened, depending upon whether or not the photoresist is negative or positive photoresist, respectively.
The pattern that is projected onto the photoresist layer is contained on a mask that is placed within the photolithography exposure tool. A mask, also referred to as a reticle, is placed between the illuminating light and the photoresist. The reticle is typically formed from patterned chromium placed on glass or quartz. The pattern is transferred onto the photoresist by projecting an image of the reticle onto the photoresist using an exposing radiation.
In many applications, the reticle is covered by a pellicle. A pellicle is a thin film of optical grade polymer that is stretched on a frame and secured to the reticle. The pellicle's purpose is to prevent airborne dirt from collecting on the mask and acting as an opaque spot. During the exposure, the dirt is held out of the focal plane and does not print on the wafer.
With the need for smaller critical dimensions, photolithography technology has evolved into using extreme ultraviolet (EUV) exposure radiation that has a smaller wavelength. One wavelength that is becoming popular is 157 nm. Unfortunately, current pellicles are formed from a material that are either not sufficiently transparent to radiation at 157 nm or does not have sufficient durability under these process conditions. Therefore, in many applications, reticles are manufactured without pellicles. This causes increased risk to contamination of the reticle.
Moreover, after the reticle has been manufactured, the reticle must be transported from the manufacturer to the semiconductor fabrication facility (known as a "fab"). This transport process increases the risk of contamination. Currently, reticles are stored and shipped in reticle containers without any devices specifically designed to remove particles from the environment in which they are enclosed.